This disclosure relates to wireless communication systems.
For battery-powered wireless communication terminals, energy and data rate constitute the fundamental resources. The energy consumption in a terminal is proportional to the electric charge drawn from the battery and will thus determine the amount of time that the terminal can operate on a single battery charge. Data rate is directly related to the quality of service perceived by the user. For example, in a packet data application, the average data rate or throughput will determine the latency in delivering any fixed amount of data. In audio or video applications, the available data rate will determine the degree of source data compression and thus the quality of received sound or picture.
In CDMA, multiple transmitters operate within the same frequency band, separated by the use of near-orthogonal spreading codes. However, the separation of signals received from different CDMA transmitters is often imperfect. After de-spreading one user""s signal in the receiver, other users"" signals will appear as additive white noise. Therefore, terrestrial wireless communication systems based on CDMA often employ sophisticated transmit power control on the reverse (mobile-to-base) link to mitigate the effects of the so-called xe2x80x9cnear-farxe2x80x9d problem. The problem arises when a base station is in communication with multiple mobile stations located at different distances from the base station. The presence of multiple signals arriving at the base station antenna simultaneously also causes an effect known as multipath. Signals that are in phase will add while signals out of phase will subtract.
The difference in power level received at the base station from a xe2x80x9cnearxe2x80x9d mobile and a xe2x80x9cfarxe2x80x9d away mobile can be large because of the high path loss associated with terrestrial radio propagation. In addition, shadowing effects as well as rapid fading caused by multipath propagation will further increase the variation in received power. The multipath fading is caused by a variation of the amplitude or relative phase of one or more of the frequency components in the received signal. In particular, multipath fading may result in the received power falling 20-30 dB below the average level, with successive minima occurring roughly every half of the carrier frequency wavelength. Consequently, with fixed mobile transmit power, the signal transmitted from a mobile located close to the base station may seriously degrade the quality of the signal received from another mobile located farther away from the base station.
Similar considerations apply to the forward (base-to-mobile) link. Although the handoff process in wireless networks attempts to ensure that, the mobile station is always in communication with the base station from which it receives the strongest signal,.short-term variations in received signal strength due to multipath propagation and variations in interference level will cause a considerable loss of system capacity unless mitigated with power control.
A well-known remedy to the near-far problem is to control the transmit power of each mobile station in such a way that all the signals arrive at the base station with approximately the same Signal-to-Interference Ratio (SIR) irrespective of where the mobile stations are located. The relevant measure of SIR is Eb/Io, where Eb is the received energy per bit from the intended mobile station and Io is the received power density from all the mobile transmitters. Furthermore, since the system capacity (i.e. maximum aggregate data rate over all simultaneous calls) is inversely proportional to the interference level, it is desirable to set the target SIR value no higher than required to ensure the desired Quality of Service (QoS). In this context, QoS is commonly measured in terms of Frame Error Rate (FER). In practice, the requested QoS and thus the target SIR may vary from one user to another.
CDMA systems generally use two fundamentally different mechanisms for power control. The first is an xe2x80x9copen-loopxe2x80x9d power control, intended to compensate for large-scale signal strength variations caused by propagation path loss and shadowing effects. Such variations can be considered as being frequency-independent. Consequently, the large-scale variations in the forward link (base-to-mobile) and the reverse link (mobile-to-base) can be considered identical, even when the two links operate in different frequency bands. In the open-loop power control, the mobile station takes advantage of this particular fact by adjusting its transmit power level autonomously in inverse proportion to the power it receives from the base station. To ensure that only large-scale variations are accounted for, the open-loop power control is based on a long-term average of the measured received power.
The second power control mechanism is a xe2x80x9cclosed-loopxe2x80x9d power control. The closed-loop control compensates for the rapid signal strength variations caused by multipath propagation, changes in interference level and sudden shadowing effects that cannot be compensated for by the slower open loop power control.
The closed-loop power control 100 includes both the base station 102 and the mobile station 104 into a feedback loop arrangement, as illustrated in FIG. 1. The system time is. divided into basic power control (PC) periods with duration TPC. In each such power control period, the base station 102 computes a short-term average of the power received from the mobile 104, as well as the power from interfering transmitters. The two measurements are then used to compute the SIR (Eb/Io) value for that period, and the computed SIR value 106 is compared to the target SIR value 108. Based on this comparison, the base station 102 computes a suitable power correction command 110, which is then transmitted back to the mobile 104 over the forward link. The mobile 104 thus can adjust its transmit power once every PC period.
The power correction commands are not protected by error correction coding to maintain the lowest possible delay in the power control loop. Moreover, in order to minimize the forward link capacity loss due to correction commands, it is desirable to encode each command as a single bit. Depending on the value of the received PC bit, the mobile station 104 may either increase or decrease its transmit power level by a predetermined amount, referred to as the PC step size.
A wireless transmitter capable of selecting a data rate on a frame-by-frame basis is described. The average data rate determines the average transmitted energy per bit of data, and thus, the energy consumption of the transmitter.
The wireless transmitter includes a data rate controller operating to predictively determine a traffic channel data rate for a frame of data using previously transmitted bit energy levels of a traffic channel and a target data throughput. The data rate controller includes a selector, a bit energy computer, a predictor, an estimator, and a comparator.
The selector selects a set of normalized thresholds using a target data throughput. The bit energy computer computes transmitted bit energy on the traffic channel based on a traffic channel signal power and data rate. The predictor predicts the transmitted bit energy in the next frame period using previously transmitted bit energy levels. The estimator is configured to compute a statistical distribution of average bit energy in the frame period using the predicted bit energy. The comparator determines the data rate on the traffic channel using the normalized thresholds, the predicted average bit energy and its statistical distribution.